As is well known, a non-volatile memory cell is read by means of a current-to-voltage conversion, suitably biasing the cell in a way that, when the stored information has a first logic value of "1", the cell is able to draw current.
In particular, for a proper memory cell reading operation, the current drawn by the cell should be greater than a preset value, termed the reference value Iref, which is usually a function of the supply voltage Vcc.
On the other hand, where the information has a second logic value of "0", the current drawn should be less than the reference value Iref.
The latest generation memory devices use values of the supply voltage Vcc ever so much smaller that a proper reading of memory cells becomes increasingly more of a problem.
In fact, discriminating between an erased cell (first logic value "1") and a written cell (second logic value "0") is only possible if the erased cell is able to draw a larger current than the reference value Iref, and this can only be if the voltage applied to the gate terminal of the cell in the reading phase, which is normally the same as the supply voltage Vcc, is higher than the cell threshold voltage.
For proper performance of the reading operation, there are available a number of circuit modifications which are primarily concerned with distributing the values of the threshold voltages Vth of the erased memory cells. The highest value of a threshold voltage Vth for an erased cell currently sets in the 2.5V range.